Highly substituted carboxyalkyl celluloses of the type described above have been described only comparatively rarely in the literature. Degrees of substitution of greater than 1.5 have only been obtained by multiple reactions on a laboratory scale in basic research. For a degree of substitution of at most 2.6, M, Bouttemy (Bull. Soc. Chim. France 1960, 1750-4) required, per mole of cellulose, from 75 to 170 moles of monochloroacetic acid which were added in approximately 10 successive etherification reactions. The selectivity levels obtained amounted to only 3.5% and 1.5%.
The production of carboxymethyl celluloses or their sodium salts with DS-values of from 1.5 to 2.0 in a multistage process, which is said to have advantages over a single-stage process, is reported in Chemical Abstracts 52 (1958) 15901 a. In this process, a very small quantity of chloroacetic acid (approximately 0.8 mole per glucose unit) has to be used in each individual stage and the crude product of each stage isolated and carefully purified before it is delivered to the next etherification stage.
None of these hitherto proposed methods for adjusting high degrees of substitution can be carried out economically on an industrial scale. They are the outcome of basic laboratory research work in which the quantities used, the reaction conditions and the reaction times are not crucial factors.